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    SubjectRe: [patch 00/10] [RFC] SLUB patches for more functionality, performance and maintenance
    On Mon, 9 Jul 2007, Mathieu Desnoyers wrote:

    > > > Yep, I volountarily used the variant without lock prefix because the
    > > > data is per cpu and I disable preemption.
    > >
    > > local_cmpxchg generates this?
    > >
    > Yes.

    Does not work here. If I use

    static void __always_inline *slab_alloc(struct kmem_cache *s,
    gfp_t gfpflags, int node, void *addr)
    void **object;
    struct kmem_cache_cpu *c;

    c = get_cpu_slab(s, smp_processor_id());
    object = c->freelist;
    if (unlikely(!object || !node_match(c, node)))
    return __slab_alloc(s, gfpflags, node, addr, c);

    if (cmpxchg_local(&c->freelist, object, object[c->offset]) != object)
    goto redo;

    if (unlikely((gfpflags & __GFP_ZERO)))
    memset(object, 0, c->objsize);

    return object;

    Then the code will include a lock prefix:

    3270: 48 8b 1a mov (%rdx),%rbx
    3273: 48 85 db test %rbx,%rbx
    3276: 74 23 je 329b <kmem_cache_alloc+0x4b>
    3278: 8b 42 14 mov 0x14(%rdx),%eax
    327b: 4c 8b 0c c3 mov (%rbx,%rax,8),%r9
    327f: 48 89 d8 mov %rbx,%rax
    3282: f0 4c 0f b1 0a lock cmpxchg %r9,(%rdx)
    3287: 48 39 c3 cmp %rax,%rbx
    328a: 75 e4 jne 3270 <kmem_cache_alloc+0x20>
    328c: 66 85 f6 test %si,%si
    328f: 78 19 js 32aa <kmem_cache_alloc+0x5a>
    3291: 48 89 d8 mov %rbx,%rax
    3294: 48 83 c4 08 add $0x8,%rsp
    3298: 5b pop %rbx
    3299: c9 leaveq
    329a: c3 retq

    > What applies to local_inc, given as example in the local_ops.txt
    > document, applies integrally to local_cmpxchg. And I would say that
    > local_cmpxchg is by far the cheapest locking mechanism I have found, and
    > use today, for my kernel tracer. The idea emerged from my need to trace
    > every execution context, including NMIs, while still providing good
    > performances. local_cmpxchg was the perfect fit; that's why I deployed
    > it in local.h in each and every architecture.

    Great idea. The SLUB allocator may be able to use your idea to improve
    both the alloc and free path.

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      Last update: 2007-07-10 01:11    [W:2.578 / U:0.732 seconds]
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